1. Field of the Invention
This invention relates generally to electronic circuits designed to supply audio frequency power to a load and, more particularly, to high efficiency bipolar power amplifier circuits which can be fabricated on a monolithic chip in what is commonly known as an integrated circuit.
2. Description of the Prior Art
In the past, several prior art types of circuits have been widely used as compact power amplifiers. One prior arrangement used series connected NPN/PNP transistors, with the load connected to the emitters at the midpoint of the complementary pair. The base-emitter junctions of the prior art circuit were slightly forward biased to reduce crossover distortion and if the transistor Betas were similar then half of the supply voltage appeared at the output terminal and each transistor could alternatively be driven to V.sub.CE(SAT). However, there were two drawbacks to this type of prior art complementary pair power amplifier: the PNP transistor was generally to large and could not be readily integrated into monolithic semiconductor chip and the high frequency output of the PNP transistor was, inherently, lower than its NPN counterpart which often created design problems.
Another prior art type of audio output circuit used two NPN transistors connected as a Darlington pair in the output stage. Higher power output could also be obtained with this prior art circuit with the use of a series resistor in the V+ connection to the driver transistors which resistor is equal to several times the load resistance. A large external capacitor was also connected from the output amplifier terminal to the driver end of the series resistor. This is a so called Bootstrap connection which, when properly designed, will produce output voltage swings in excess of one half of the V+ supply. One of the objections to Boot-strapping is the need for the external capacitor which increases the cost and bulk of the amplifier. Power loss in the driver dropping resistor was also a drawback.
U.S. Pat. No. 3,938,053 filed Sept. 19, 1974, by Menniti et al describes still another Class-B type of power amplifier designed for a monolithic IC chip. This circuit uses an NPN output transistor whose base is driven by an NPN control transistor through an intermediate pair of PNP transistors whose emitters are connected to the V+ supply. The V+ supply is also connected to the collector of the NPN output transistor. The base terminals of the intermediate PNP transistors are connected together and the common base lead is connected to the collector lead of one of the PNP pairs and to the collector terminal of the NPN control transistor. The emitter of the NPN control transistor is connected to the output load terminal which is also connected to the emitter of the NPN output transistor. The collector terminal of the other PNP intermediate transistor is connected to the base terminal of the NPN output transistor. On positive half-cycles of the signal to be amplified, this intermediate PNP transistor saturates prior to saturation of the input transistor, thereby increasing the output voltage at the load terminal.
The above described prior art amplifier, while not attaining the voltage swing across the load equal to a power amplifier circuit having a Bootstrap feature, does avoid the necessity of a large off-chip capacitor. The direct connection between the emitter of the control transistor and the output terminal causes HF response problems under some load conditions. The overall current gain of this prior art circuit is only about 10,000.
The foregoing discussion of prior art power amplifier circuits demonstrates that a need exists for an improved power amplifier circuit which can be fabricated in an IC chip, which does not require expensive or bulky external components and which is inherently stable under all load, input, supply, and high frequency output situations and which due to its high input impedence has an overall current gain of at least 100,000.